In a television receiver, a monitor apparatus and the like, for example, a chroma demodulation system is provided for separating a luminance signal (Y-signal) and a chroma signal (C-signal) from an input composite video signal, and further demodulating a color difference signal from the chroma signal. In recent years, the above-mentioned chroma demodulation system is constituted by a digital circuit, so that the chroma demodulation carried out by a digital signal process has been proposed and also executed.
The above-mentioned digital chroma demodulation system executes, for example, a Y/C separation for separating into a luminance signal and a chroma signal after converting an input analog composite video signal into a digital signal, and also performs the chroma demodulation process on the separated chroma signal and consequently generates color difference signals Cb, Cr. As a result, the digital chroma demodulation system outputs the luminance signal and color difference signals, which are necessary for a color image display.
Then, as a system clock to operate the above-mentioned digital chroma demodulation system, for the necessity of extracting the color difference signal, an operation is carried out for synchronizing with a color burst signal which is superimposed on the composite video signal. In this case, the system clock for the digital chroma demodulation system is generated by carrying out the operation so as to lock a PLL circuit in synchronization with the color burst signal extracted from the composite video signal.
In addition, the frequency of the system clock for the digital chroma demodulation system as mentioned above is set to 4 fsc which is equal to 4 times the frequency fsc of the color burst signal, in many cases. This is because when it is considered that sampling is executed so as to obtain the quality which is said to be sufficiently high with regard to the luminance signal and color difference signal in the digital chroma demodulation system, as the frequency obtained after the multiplication of the frequency fsc of the color burst signal, 4 fsc is the minimum necessary frequency.
A block diagram of FIG. 7 schematically shows one example of the digital chroma demodulation system based on the above-mentioned configuration.
Here, as the chroma demodulation system, a system is known which is designed so as to be decodable, corresponding to the inputs of not only the composite video signal through the particular one television system but also the composite video signals through a plurality of kinds of television systems. It is designed such that it can execute the decoding process, for example, corresponding to even the input of any composite video signal of various NTSC systems, PAL systems, and SECAM and the like.
So, the digital chroma demodulation system shown in FIG. 7 is explained under the assumption that it also employs the configuration corresponding to the multi-input of the above-mentioned composite video signal.
A system clock CLK is inputted to a digital chroma demodulation system 100 shown in FIG. 7. An A/D converter 101, a Y/C separation circuit 103 and a chroma demodulation circuit 104, which constitute the digital chroma demodulation system 100, are operated at the timing in accordance with this system clock CLK. Here, this system clock CLK is outputted from the PLL circuit included by the chroma demodulation circuit 104. This PLL circuit generates and outputs the system clock synchronized with the color burst signal by operating so as to be locked corresponding to the color burst signal of the input composite video signal. Then, the frequency of the system clock CLK in this case is assumed to be 4 fsc, as mentioned above.
For example, when the composite video signal of an NTSC system is inputted, since the frequency fsc of the color burst signal is 3.58 MHz, a system clock frequency is 14.32 MHz (=4×3.58 MHz). In addition, when the composite video signal of a PAL system is inputted, the frequency fsc of the color burst signal is 4.43 MHz. Thus, the system clock frequency is 17.72 MHz (=4×4.43 MHz).
The composite video signal which is to be inputted to the digital chroma demodulation system 100 is firstly inputted to the A/D converter 101. The A/D converter 101 performs the A/D conversion on the input composite video signal at the operation timing based on the system clock CLK of 4 sfc and outputs the digital composite video signal to a terminal T1 of a switch circuit 102 and the Y/C separation circuit 103.
The Y/C separation circuit 103 performs the operation as a comb-shaped filter formed as, for example, a digital circuit, on the input composite video signal, and consequently separates into the luminance signal (Y-signal) and the chroma signal (C-signal). The luminance signal is outputted to a terminal T2 of the switch circuit 102, and the chroma signal is outputted to the chroma demodulation circuit 104.
The chroma demodulation circuit 104 performs a decoding process on the input chroma signal through a digital signal process and outputs the color difference signals Cr and Cb.
The switch circuit 102 carries out the switching so that a terminal T3 is alternatively connected to the terminal T1 or terminal T2. In a normal case, the terminal T3 is connected to the terminal T2.
Consequently, the digital chroma demodulation system 100 outputs the luminance signal and the color difference signals Cr, Cb, which are extracted from the input composite video signal.
Here, the switch circuit 102 is included corresponding to the fact that the digital chroma demodulation system 100 shown in FIG. 7 is configured corresponding to the multi-input of the composite video signal.
In the case of corresponding to the multi-input of the composite video signal, there maybe a case that the television system of the input composite video signal to be inputted is switched, for example, from the NTSC system to the PAL system.
In this way, when the television system of the composite video signal to be inputted is switched and changed the frequency of the color burst signal or when the burst signal is not inserted into the composite video signal to be inputted, the digital chroma demodulation system 100 enters the determining operation for the television system.
The period while the determination of the television system is performed as mentioned above is at the situation that the system clock CLK complying with the input composite video signal can not yet be generated. Thus, since the Y/C separating and chroma demodulation processes can not be correctly performed, the luminance signal and the color difference signals Cr, Cb can not be outputted.
So, in the period while the determining operation for the television system is performed, in the switch circuit 102, the terminal T1 and the terminal T3 are connected. Consequently, instead of the luminance signal, the composite video signal (CVBS signal) after the A/D conversion is directly outputted to a video signal processing system at a later stage. Thus, for example, although a black and white image screen, an image display output based on a video signal can be kept.
Here, the determining operation for the television system as mentioned above is performed as follows in brief.
For example, in assuming that the composite video signal of the different television system from the previous television system or the composite video signal into which the color burst signal is not inserted is started to be inputted to the digital chroma demodulation system 100.
At the above-mentioned situation, on the digital chroma demodulation system 100 side, the frequency of the system clock is switched to the frequency (4 fsc) corresponding to the television system which is pre-assumed to be inputted every several times of a vertical scan period in order to determine the television system on which the input composite video signal is based.
For example, at first, 14.32 MHz that is the frequency of 4 fsc corresponding to the NTSC system is set for the frequency of the system clock CLK, and the digital chroma demodulation system 100 is operated. Due to this operation, as mentioned above, the PLL circuit included by the chroma demodulation circuit 104 detects whether or not it can be locked to the color burst signal. Here, if the composite video signal to be inputted is based on the NTSC system, the PLL circuit obtains the converging operation so that it is locked to the color burst signal, and determines that the composite video signal presently inputted is based on the NTSC system. Hereafter, on the basis of the system clock CLK of this 14.32 MHz, the digital chroma demodulation system 100 is operated.
On the contrary, even after the elapse of the vertical scan periods corresponding to several times, if the PLL circuit can not obtain the situation that it is locked to the color burst signal, next, for example, the frequency of the system clock is switched to 4 fsc=17.72 MHz corresponding to the PAL system. Similarly to the above-mentioned case, the PLL circuit detects whether or not it can be locked to the color burst signal, within the vertical scan periods corresponding to the several times.
Here, according to the explanation with reference to FIG. 7, in the period until the determining operation for the television system is finished, from the digital chroma demodulation system 100, the composite video signal converted into the digital signal by the A/D converter 101 is outputted directly through the switch circuit 102. Then, it becomes the situation that the image displaying through this composite video signal is being performed.
Then, at this time, as mentioned above, the switching of the system clock frequency based on the television system is performed. However, the system clock frequency that is 4 fsc is, for example, 14.32 MHz in the NTSC system, and 17.72 MHz in the PAL system. Namely, when the determining operation of the television system is actually being performed, even at least between the NTSC system and the PAL system, the system clock frequency is changed by 20% or more every each vertical scan periods corresponding to the several times.
Then, in response to the fact that the system clock frequency is greatly changed as mentioned above, for example, the sampling point, sampling frequency and the like in the A/D converter 101 are changed, which consequently brings about the change even in the image viewed by outputting the A/D-converted composite video signal for display.
More specifically, the appearance frequency characteristic is changed so as to be extended or dropped. In addition, the appearance of the moire portion caused by the returning of the portion of a high-frequency signal may be also changed. Then, the change in the image as mentioned above is frequently changed in the periods corresponding to each vertical scan periods corresponding to the several times. Thus, the displayed image becomes visually degraded.
The above-mentioned phenomenon becomes the serious problem in the case that, in particular, for example, the color burst signal is not inserted and the black and white composite video signal is inserted.
In other words, the determining operation for the television system as mentioned above is carried out by determining whether the color burst signal is in the locked state or not. Thus, if the color burst signal is not inserted as the black and white composite video signal, the switching of the system clock frequency is continuously repeated. Hence, in this case, the visually degraded image, which is frequently changed, is continuously displayed.
In this way, in the digital chroma demodulation system configured corresponding to the multi-input of the composite video signal, for example, the inevitable execution of the switching of the system clock frequency in association with the determining operation for the television system causes the disturbance in the display image when the system clock frequency is switched.